FB331

So the issue is that once every month my 84 gsl will idle at 1100 rpm when I come to a red light. I'll be sitting there enjoying mysel and I look down and see my idle is 1100 instead of the solid 750 it usually is. Then I will drive to another stop and it will be 750.

So my question being what would cause my car to randomly choose to idle at 1100 instead of my regular 750

And this will only happen 1 every month probably for a hour or so at a time and then be normal??

Any input is welcomed